fixes #4186

compiler/ccgexprs.nim

@@ -611,7 +611,7 @@ proc genEqProc(p: BProc, e: PNode, d: var TLoc) =
   assert(e.sons[2].typ != nil)
   initLocExpr(p, e.sons[1], a)
   initLocExpr(p, e.sons[2], b)
-  if a.t.callConv == ccClosure:
+  if a.t.skipTypes(abstractInst).callConv == ccClosure:
     putIntoDest(p, d, e.typ,
       "($1.ClPrc == $2.ClPrc && $1.ClEnv == $2.ClEnv)" % [rdLoc(a), rdLoc(b)])
   else:

tests/ccgbugs/tclosureeq.nim

@@ -0,0 +1,19 @@
+discard """
+  output: '''true
+true'''
+"""
+
+# bug #4186
+type
+  Predicate[T] = proc(item: T): bool
+
+proc a[T](): Predicate[T] =
+  return nil
+
+proc b[T](): Predicate[T] =
+  return a[T]()
+
+echo b[int]() == nil  # ok
+
+let x = b[int]()
+echo x == nil     #won't compile